http://arxiv.org/abs/1405.3516
A number of radio-loud ultra cool dwarf (UCD) stars exhibit both continuous broadband and highly polarized pulsed radio emission. In order to determine the nature of the emission and the physical characteristics in the source region, we have made multi-epoch, wideband spectral observations of two UCD stars, TVLM 0513-46 and 2M 0746+20. We combine these observations with previously published and archival radio data to fully characterize both the temporal and spectral properties of the radio emission. The continuum spectral energy distribution and fractional polarization can be well modeled using gyrosynchrotron emission with mean magnetic field $B\sim10-70$ G, mildly relativistic power-law electrons of number density $n_e\sim10^{5-6}$cm$^{-3}$, and source size $R\sim 2R_s$. The pulsed emission exhibits a variety of time-variable characteristics, including large frequency drifts, high and low frequency cutoffs, and multiple pulses per period. For 2M 0746+20 we determine a main pulse period consistent with previously determined values, but for TVLM 0513-46 the radio pulse period is 30 sec longer than the optically-determined period. We modeled the locations of pulsed radio emission using an oblique rotating magnetospheric model with beamed electron cyclotron maser sources on local magnetic loops. The best-fit models have ECM beaming angles that are very narrow and aligned with the magnetic field tangent direction at the source, except for one isolated burst from 2M 0746+20. For TVLM 0513-46, the best-fit rotation axis inclination is nearly orthogonal to the line of sight. For 2M 0746+20 we found a good fit using a fixed inclination $i=36^{\circ}$, as recently determined from optical observations. For both stars the ECM sources are located near the feet of intense, isolated magnetic loops with scale heights $1.2R_s-2.7 R_s$ and surface fields 2.2 — 2.5 kG.
C. Lynch, R. Mutel and M. Gudel
Thu, 15 May 14
17/55
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